Back-of-envelope estimation

After you clarify what you are building, you size it. Not precisely, roughly. The difference between an app with a thousand users and one with a billion is not a detail, it is a different design: one runs on a single database, the other needs caches, replicas, and sharding. Estimation is how you find out which world you are in.

The good news is that the math is deliberately sloppy. You round hard, you aim for the order of magnitude (is it hundreds, thousands, or millions per second?), and you move on. Nobody wants six decimal places. They want to see that your design matches the scale you are actually facing.

This unit gives you the three numbers you estimate most, requests per second, storage, and the read-to-write ratio, plus a short list of latency facts that explain why caches and CDNs work at all.